Proton NMR, luminescence and electrochemical study of 18-membered Schiff-base macrocyclic lanthanum(III) complexes

Abstract

An NMR, electrochemical and photophysical study has been made of lanthanide(III) complexes with a 18-membered hexadentate Schiff-base N4O2 polyoxaazamacrocycle L1 derived from the condensation of 2,6-bis(2-aminophenoxymethyl)pyridine and 2,6-diformylpyridine, as well as their interaction with the chelating agents 1,10-phenanthroline and 2,2′-bipyridine. Separation of the contact and pseudocontact contributions to the observed 1H NMR lanthanide-induced shift values point to an isostructural series of [Ln(L1)][ClO4]3 (Ln = La–Eu, except Pm) in acetonitrile. The interaction of the lanthanum(III) complex with 1,10-phenanthroline and 2,2′-bipyridine was demonstrated by the use of different spectroscopic techniques and the stability constant for the 1,10-phenanthroline adduct was calculated from 1H NMR data. Upon excitation through the metal excited levels, the terbium(III) complex displays the typical luminescence originating from the excited 5D4 level, while the adduct with 1,10-phenanthroline presents emission upon excitation through the ligand bands. The reaction of 2,6-bis(2-aminophenoxymethyl)pyridine and 2,6-diformylpyridine and the perchlorate of GdIII or TbIII in ethanol solution yielded a reduced macrocycle L2 instead of the expected complexes of L1. Electrochemical studies in acetonitrile solution showed that the reduction of the imine groups is favoured when the ionic radius of the lanthanide(III) ion decreases.